The present invention relates to a method and apparatus for construction of a ladder and more particularly to a method and apparatus for treating ladder rungs for forming assembly with ladder stiles or rails, the rungs generally being of a metallic material and in one advantageous embodiment of the present invention of an extruded aluminum alloy material.
It is known in the art of ladder manufacture to form metallic ladders, such as extruded aluminum alloy ladders, by heating opposed, extruded aluminum alloy ladder rung extremities in annular induction heating coils through which the extremities of a rung are inserted by moving each rung in a first direction along its longitudinal axis until a preselected area of the rung extremity extends into and is surrounded by the annular induction heating coil, the rung extremity being inductively heated quickly to a range of from 600.degree. F. to 1100.degree. F. over a very brief period of time ranging from one-half second to ten seconds so as to heat the inserted end portion without migration of the heat to the central portion of the rung, attention being directed to U.S. Pat. No. 4,766,664, issued to Joseph C. Benedyk on Aug. 30, 1988. Such an annular heating arrangement presents complex problems when adapted to the mass production of ladder rungs for subsequent forming assembly with ladder stiles or rails, requiring a first set of a plurality of such annular induction heating coils in proximate, aligned, side-by-side linear plane relation with concomitant complex problems of alignment and the moving of the ladder rungs first in one direction along the lines of their longitudinal axes for a preselected distance into such coils to treat corresponding extremities of the rungs and then moving the rungs in an opposite direction again along the lines of their longitudinal axes through a second opposed set of a plurality of annular induction heating coils in proximate, aligned, side-by-side linear plane relation. Not only is such an arrangement comparatively complex, requiring equally complex and expensive machinery, but the arrangement tends to interrupt progressive, aligned production flow patterns with the proximately positioned annular induction heating coils of each set creating problems of undesirable magnetic flux distortion and interference with the uniform and controlled heating of preselected rung areas-particularly in those instances where the rungs are of non-symmetrical cross-section along their longitudinal axes. Further, in the past arrangements, fine mechanical adjustments have been difficult, probably because of the rapid heating period. This rapid acceleration of heating does not allow the "fine tuning" necessary to prevent the melting of rungs of non-symmetrical cross-section along their longitudinal axes.
In accordance with the present invention, an economical, efficient and comparatively continuous method of treating ladder rungs for subsequent forming assembly with ladder rails and a unique structure to accomplish such inventive method are provided which inventive method and apparatus are straightforward and comparatively uncomplex in carrying out the several steps of the method and with the novel structural arrangement being readily and efficiently constructed with a minimum of parts and with a minimum of required space. Further, the present invention allows the use of high strength, extruded metallic alloys, particularly extruded aluminum alloys with a minimum of elongation thereof and with the ductility and formability of the raw extrusions not being critical. Further, the present invention avoids past problems of treating control at desired elevated production rates and the frequent "tuning" of the process and the equipment involved, as would be required with a multiple annular induction heating coil process wherein each annular coil presents a separate treating window for each ladder rung extremity. In carrying out the process of the present invention, all of the ladder rungs to be treated "see" or progressively pass through the same treating window and uniformity of treatment is readily and easily obtainable with a minimum of repair and maintenance required and with adjustment of residence time and temperature being equal for all rungs treated. Moreover, the present invention allows for rapid adjustment to treating batches of rungs of different length and of different cross-sections along the longitudinal axes, such as rungs of circular cross-section along the longitudinal axes and rungs of non-symmetrical cross-section along the longitudinal axes, such as triangular or "D" shaped rungs, all types of rungs being readily acceptable in the inventive process and structure. Furthermore, the present invention provides for careful and accurate control of temperature rises to avoid rung burning or melting, providing a controlled rung soaking process, permitting automatic power and temperature adjustments during batch rung treatment. In addition, the problem of heat migration to rung centers, which has been of major concern in past processes, requiring quick temperature elevation and subsequent quenching, has been avoided in the herein inventive process and structure, with only minimal and non-consequential heat migration to rung centers occurring without effecting permanent metallurgical changes which undesirably reduce rung hardness at the rung centers where such hardness is desired.
Various other features of the present invention will become obvious to one skilled in the art upon reading the disclosure set forth herein.